Trimming lets you make permanent adjustments to the flight surfaces, so that the aircraft can fly straight without any input. For example, if your aircraft is rolling to one side all the time, you can apply trimming to balance that out and make the plane fly straight without having to keep the stick pushed to the other side.
It's not completely necessary, and I usually never use trimming except for rare cases, but I suppose it can help.
EDIT: Ah, damnit Arclight!
